Recently, the storage battery devices including a lithium ion storage batteries or the like has been used in many situations. For example, a storage battery device is used as the driving source for an electric car, used for peak cutting carried out to reduce power consumption at the time period of great power demand on a demand side such as a household, a store, or a commercial establishment, used for load shifting carried out to consume power at the time period of small power demand on the demand side, or used for stabilizing unstable renewable energy outputting on a system side such as a power company or a heavy electric machinery company. In the storage battery device used for any of such purposes, a storage battery device that has a large capacity is necessary. The storage battery device of the large capacity can be achieved by connecting a plurality of storage battery cells in series.
Characteristics of a storage battery, such as effective power storage amount, are known to greatly deteriorate due to natural discharging, long-time use, or a charging/discharging cycle. Storage battery cells included in the storage battery does not degrade evenly each cell. Therefore, in the storage battery, storage battery cells that have undergone capacity degradation and storage battery cells which have not undergone capacity degradation are frequently mixed. In such a case, the overall characteristics of the storage battery are determined by the characteristics of the storage battery cells which have undergone capacity degradation.
Under these circumstances, a storage battery device that equally uses storage battery cells for the purpose of preventing shortening of a life has been offered (refer to Patent Literatures 1 and 2).
Patent Literature 1 describes a power supply system that outputs a voltage from each of a plurality of storage battery cells connected in series.
FIG. 1 is a circuit diagram illustrating the power supply system described in Patent Literature 1.
In FIG. 1, storage battery D includes storage battery cells B1, B2, and B3 connected in series.
Device A is connected to terminal 1, device B is connected to terminal 2, device C is connected to terminal 3, and return wire R is connected to terminal 4. Devices A, B, and C are activated by power discharged from storage battery D. Balance circuit 10 is connected to terminals 1 to 4. The output voltages of storage battery cells B1 to B3 are made uniform by exchanging energy among storage battery cells B1 to B3.
In a power supply system configured by removing balance circuit 10 from the power supply system illustrated in FIG. 1, usually, currents flowing through storage battery cells B1 to B3 are not uniform, and the output voltages of the storage battery cells are not uniform.
FIG. 2 is a graph illustrating the changes of the output voltages of storage battery cells B1 to B3 when power is discharged from storage battery D from an initial state where storage battery cells B1 to B3 are fully charged in the power supply system configured by removing balance circuit 10 from the power supply system illustrated in FIG. 1.
FIG. 3 is a graph illustrating the changes of the output voltages of storage battery cells B1 to B3 when power is discharged from storage battery D from an initial state where storage battery cells B1 to B3 are fully charged in the power supply system illustrated in FIG. 1.
The changes of the output voltages of storage battery cells B1 to B3 illustrated in FIG. 2 are different from one another, while the changes of the output voltages of storage battery cells B1 to B3 illustrated in FIG. 3 are similar.
Discharging permission time illustrated in FIGS. 2 and 3 represents time from a discharging start to a discharging inhibition voltage of, among storage battery cells B1 to B3, storage battery cell B3 the output voltage of which reaches the discharging inhibition voltage shortest in time from the discharging start. The discharging permission time illustrated in FIG. 2 is longer than that illustrated in FIG. 3.
Patent Literature 2 discloses a charging/discharging control circuit that causes, when a storage battery including a plurality of storage battery cells connected in series is charged or discharged, the output voltages of the storage battery cells to become uniform.
FIG. 4 is a block diagram illustrating a device that includes the charging/discharging control circuit described in Patent Literature 2.
In the device illustrated in FIG. 4, when switch 13 is ON while switch 14 is OFF, charging/discharging control circuit 20 charges storage battery 12 by using DC power source 15. When switch 14 is ON while switch 13 is OFF, power is charged from storage battery 12, and the discharged power is consumed by load 11.
Charging/discharging control circuit 20 illustrated in FIG. 4 compares the output voltage of each storage battery cell with the average among of the output voltages of all the storage battery cells. Then, charging/discharging control circuit 20 switches switches 13 and 14 ON/OFF according to the result of the comparison, divides some of currents discharged from the respective storage battery cells to flow into charging/discharging control circuit 20, thereby causing the output voltages of the storage battery cells to become uniform.
Thus, by causing the output voltages of the storage battery cells to become uniform, rapid degradation of storage battery cells that have undergone capacity degradation is prevented and the battery life of storage battery 12 as a whole can be prolonged.